In many heavy industries and in mines, there is much dust in the air. Federal standards for early warning of fires in mines use an obscuration standard rather than particle ionization or particle reflection technology. Unfortunately most optical devices are expensive because of their short life in dusty and moist environments. Some smoke detection devices use high levels of radioactive materials to avoid dust accumulation and are hazardous because of the radiation.
Smoke monitoring is essential in many industries in which smoke is generated without much carbon monoxide, which is commonly monitored. For example, compressor overheating, conveyor belt slippage, electrical equipment faults and other types of overheating incidents, particularly involving plastics, can produce a great deal of smoke but minimal amounts of carbon monoxide. In many industries using heavy equipment, the above incidents may generate thick, choking smoke, while there is no activation of a conventional temperature of carbon monoxide sensor and alarm. Compared with point heat (temperature) sensors, carbon monoxide sensors are invaluable for early detection of spontaneous combustion of coal and wood. However, smoke monitoring is necessary to detect fires with low heat and low output of carbon monoxide.
Conventional smoke detectors cannot be used in dusty conditions because of false alarms and clogging with dust. The U.S. Department of the Interior, Bureau of Mines, tested technologies using light scattering and ionization, as well as air pumps to convey the gas sample to the detection device, reference IC 9311. Unfortunately, although these devices detected some actual smoke-forming events, false alarms were frequent.
Additionally, acetylene and hydrogen gases interfere with carbon monoxide sensors. Moreover, most carbon monoxide sensors are temperature sensitive, and there are maintenance problems involved in establishing an ambient level of carbon monoxide. Conventional carbon monoxide sensors require costly calibration gas, have a short cell life, and these sensors are inclined to drift.
U.S. Department of the Labor, Mine Safety and Health Administration (MHSA) regulations permit either a carbon dioxide or a smoke sensor for fire monitoring of underground electrical sites. Among the federal 30 CFR regulations relating to smoke monitors are:
75.340 (a) Underground electrical installations, monitor intake air for carbon monoxide or smoke; 5% per meter smoke alarm level; functional test every 31 days. Although carbon monoxide or smoke sensors are permitted, the sensitivity to hydrogen gas discounts carbon monoxide if battery chargers are involved.
75,344 Compressors: specifies carbon monoxide or smoke.
75,351. Atmospheric monitoring systems: smoke alarm set at 0.05 per meter (5% obscuration); electrical equipment, such as transformers, battery chargers, substations, rectifiers, or water pumps, site at least on sensor (carbon monoxide or smoke) 50-100 feet in the direction of air flow; functional testing every 31 days. Only one alarm level is specified for smoke, compared with two alarm levels for CO, although the analog smoke monitor can control visual warning devices in the same manner as a carbon monoxide sensor.
The MSHA standard prescribes a light absorption value that does not correlate easily with ionizing figures. Light obscuration has little correlation with the degree of particle ionization. Given a level of smoke obscuration from a real fire, of approximately a 5% alarm level, the actual degree of particle ionization produced depends greatly upon the source of the smoke, including the intensity of the fire, the materials involved, the air velocity, oxygen concentration, and the like. These variables are not normally known, controlled, or monitored. Therefore, inferring the level of obscuration corresponding to a measured ionization level is highly complex, and is certainly not a linear and faithful relationship. The dilemma the user faces is that the ionization warning and alarm levels chosen can result in noncompliance with the optical obscuration standard under the worst case combustion conditions. Otherwise, the ionization alarm levels must be set so low as to be impractical, resulting in false alarms and sensitivity to dust.
Moreover, most smoke detectors are just that--"smoke detectors," and merely indicate an alarm or no alarm (smoke or no smoke) condition. An ionizing smoke detector has no analog output, and alarms tend to be inconvenient surprises, with no prior warning of impending problems. If an ionizing smoke detector becomes contaminated with dust, generally the only option is to remove the detector from the mine, and to return the detector to the factory for repair.
Prior workers have sought to provide smoke sensors that can function in dusty environments, but all of them have some drawbacks.
Kobayashi et al., in U.S. Pat. No. 5,247,283, disclose a smoke sensor which can be used in a dusty environment and which can be tested to ensure that it is operating properly. A light beam which is emitted from a main light emitting element and is scattered in an interior sensing space is received at a main light receiving element, and an output signal responsive to the quantity of light received at the main light receiving element is processed at a signal processing circuit for sensing the presence of smoke or dust particles. A second output of an auxiliary light receiving element for the emitted light directly received from the main light emitting element and a composite output of the first and second output are compared. An alarm is generated when the level of output is other than a predetermined amount. In this case, dust interference is eliminated by providing a net. There is no indicator of smoke other than an alarm.
Best, in U.S. Pat. No. 4,906,978, disclose an optical smoke detector of the Tyndall type which measures light scattering. In this type of detector, smoke particles are admitted into a closed smoke chamber while outside ambient light is precluded from the chamber. A light beam projected into a test zone of the chamber will strike and bounce off of dust or smoke particles in the test zone to produce scattered or diffused light which is then detected by a photo or light responsive sensor to indicate the presence of an alarm condition. In the absence of dust or smoke particles, the light responsive sensor is optically shielded from the light source. At least one of the surfaces of the smoke chamber is shaped to provide a series of parallel grooves for accumulating undesired dirt and dust in the groove crevices and for minimizing the reflection of light from that dirt and dust and toward the radiation sensing means, thereby to preclude false alarms.
Scheidweiler, in U.S. Pat. No. 4,384,488, discloses a smoke detector containing at least one smoke measuring chamber which possesses a mounting or socket plate for mounting on the ceiling of a room, and a housing enclosing the smoke measuring chamber and suspendingly secured at the socket plate. Openings in the housing permit entry of ambient air. The air entry openings can be changed to account for dust in the air. Protuberances or projections can be included in the inlet to cause deflection of the incoming or entering air. The cross-sectional area of the air entry opening can be adjusted to reduce interference from dust particles. In this case, the smoke sensing element itself is an ionization type, not optical.
Horvath et al., U.S. Pat. No. 4,269,510, disclose a smoke detector for a fire alarm comprising a nuclear radiation source possessing a substantially conical ring-shaped radiation characteristic or pattern and a radiation receiver arranged along the axis of the radiation source but externally of the direct radiation. The radiation receiver receives radiation which is scattered by smoke particles within the conical ring-shaped radiation region. To reduce the spurious radiation impinging on the radiation receiver the radiation region is bounded by elements located externally of the direct receiving region of the radiation receiver. Web means are provided which are shaped to prevent dust from collecting on their radiation receiving surfaces to minimize the effects of dust.
Malinowski, in U.S. Pat. No. 4,099,065, discloses a smoke detector operating on the principle of reflected light. Smoke inlet slots are provided between the light and photo-cell recesses which prevents accumulation of dust and dirt therebetween which could cause a false alarm.
Igarashi et al., U.S. Pat. No. 5,021,677, disclose a light scattering-type smoke detector comprising a dark box which is surrounded by a labyrinth around its outer periphery that is internally provided with a light projection element and a light receiving element. A test lamp using a light emitting diode is surrounded by light-shielding columns. The light-shielding columns each have a J-shaped configuration which constitutes part of the labyrinth surrounding the test lamp. In a normal state clean air containing no smoke particles flows into the dark box and light from the light projecting element is nor received by the light receiving element. This device uses scattered and reflected light to detect smoke.
Payton et al., in U.S. Pat. No. 3,500,450, disclose an optical device for the examination of smoke and dust laden gas consisting of two colinear tubular portions connected together by a support member. One tubular portion contains a light source and the other tubular portion contains a light-sensitive device. Smoke or dust-laden gas is arranged to pass upwardly between the tow tubular portions and thus between the light source and the photo-sensitive device. Baffles are inserted into each tubular portion to prevent dust or dirt particles form being deposited either onto the light source or onto the photosensitive device.
Tashiro et al., in U.S. Pat. No. 3,708,675, disclose a smoke detector in which air entrance and egress are located in oppositely disposed surfaces. The air flow passages are arranged so as to cause the air to flow at different velocities. When there is smoke present in the air, the velocity difference in the air flow creates a difference in static pressure that aids in introducing smoke into the casing.
Austin et al., in U.S. Pat. No. 5,170,150, disclose a smoke detector which incorporates therein means for verifying the detector's smoke sensitivity limits. This means includes positively positioning various light reflecting surfaces to provide predetermined light reflection onto light sensing means to create simulated smoke levels.
None of the above-noted smoke detectors is designed to work in a particularly dusty or dirty environment such as a coal mine. Furthermore, none of the above smoke detectors responds to the primary legal fire alarm parameter, namely, the obscuration of a beam of light. The reflected light devices bear the same ambiguity as the ionizing particle devices, namely, correlating the sensor output with the primary legal requirement, the percent obscuration of a light beam. The amount of light reflected from smoke of a given light obscuration level is influenced by the color of the smoke, the particle size and distribution, the degree of turbulence in the optical chamber and other parameters which make a direct correlation with the optical obscuration standard extremely complex, if not impossible.